A technique is known wherein when the influence of polarization still remains after the charging or discharging of a battery has been ended, an open-circuit voltage (OCV) that would be provided if the polarization was eliminated is estimated from a measured temporal variation in the voltage of the battery, and the state of charge (SOC) is estimated on the basis of a SOC-OCV characteristic curve of the battery using the estimated open-circuit voltage.
Related techniques include the techniques described in patent literatures 1 and 2.